The Section on Neuroendocrinology conducts research on the pineal gland with heavy emphasis on the biochemistry and molecular biology of the two enzymes involve in the conversation of serotonin to melatonin. This indolic product is synthesized at high levels at night in all vertebrates, due to a marked increase in the activity of the first enzyme in the serotonin - melatonin pathway, serotonin N-acetyltransferase (arylalkylamine N-acetyltransferase, AANAT, EC 2.3.1.87). The Section has determined the genetic code of the enzyme in humans, in four other mammals, a bird, and three fish, which has allowed them to analyze the regulation of mRNA encoding the enzyme and to raise antisera against the protein, which have made it possible to study the regulation of enzyme protein. These efforts have revealed that the increase in AANAT activity reflects two mechanisms and perhaps more. In all cases, the increase in AANAT activity at night is associated with an increase in enzyme protein. In some cases, such as the rat, this also requires an increase in AANAT mRNA. However this is not the case in trout and in sheep, in which the increase occurs in the absence of an change in mRNA. One mechanism through which AANAT protein increases is through inhibition of proteolysis. It is also possible that translation increases, although no evidence in support of this is present. The mechanisms which increase mRNA in the rat appear to involve cyclic AMP dependent transcription, regulated via a CRE/CCAAT regulatory element in the AANAT promoter. In addition, negative transcription factors - ICER and Fra-2 served to modulate transcription. In the chicken it appears that expression of the AANAT genes is linked to the circadian clock in the chicken pineal gland. Cloning AANAT has made it possible to prepare large amounts of AANAT protein, which have been used to study the mechanism of catalytic action, and to determine that this appears to involve the formation on the enzyme of an CoA-acetyl-arylalkylamine intermediate. Site directed mutagenesis indicates that it is highly probable that three histidines between the putative arylalkylamine binding domain and putative CoA binding site represent the catalytic domain.